Does energy efficiency result in a real decrease in energy usage? According to human history and a study from the Sandia National Laboratories, the answer is no. As the rate at which a resource is consumed becomes more efficient, demand for the resource actually increases. It's an interesting side effect of advancing technology.
You may have experienced this; I know I have. If I am able to obtain a product or service (like light bulbs
and lighting) for a certain price, I become accustomed to said price. If suddenly I am able to obtain MORE product or service for the SAME price, rather than decrease usage I will increase it such that the price I am paying remains the same. It seems to be human nature. Consider the numerous studies stating that as far back as A.D. 1700 and candle/oil/gas fueled lighting, the consumption of fuel and energy remains a relative constant. If fuel efficiency increases, consumption increases and the actual rate of resource loss remains about the same.
The leader of the study I mentioned in the first paragraph is Jeff Tsao. He noted that we behave similarly with vehicles. If we have cars that are more fuel efficient, it doesn't necessarily mean we're going to drive less. It is more likely that we will drive more until we have reached the point where we are paying about as much for gas as we were before, because we have grown accustomed to the price.
The basic thought causing this behavior is this: "My routine involves paying X for Y service. I'm accustomed to paying X for Y, so if I can pay the same X for double Y, I gain big!" We have to do more than just change our light bulbs. We have to change our thinking if we want to reap the benefits that energy efficiency can potentially provide.